Exemplary embodiments of the present invention relate to a frequency hopping method for a radio.
Radios that use a frequency hopping method are common knowledge today. This method is used to reduce transmission errors (for example as a result of fading) and also to prevent the transmitted information (voice or data) from being recovered by unauthorized third parties. However, known receivers can already be used to record the individual hops in these frequency hopping methods over a wide band and then to recover the information contained in the signal using classification methods (“de-hopping”).
Exemplary embodiments of the present invention are directed to a method that can be used to prevent this recovery of information from the received signal.
According to exemplary embodiments of the present invention, the individual frequency hops involve one or more of the following signal parameters being changed on a pseudo random basis:
1. Switching between different, related or unrelated, modulation types (e.g. QPSK, FSK, ASK, FM, . . . ).
2. Switching the bandwidth. In order to be able to switch between different bandwidths, it is advantageous for supplementary data to be inserted for the useful data at higher bandwidths. This can be accomplished using of one of the following alternatives:                oversampling the signal when transmitting analog data, such as voice—this also guarantees constant or better robustness towards transmission errors on account of the inserted redundancy;        multiplying the useful data when transmitting digital useful data—this likewise guarantees constant or better robustness toward transmission errors on account of the inserted redundancy;        inserting an information sequence that is known to the receiver, for example a pseudo random bit sequence (e.g. pseudo noise (PN) sequences) or bits of an error recognition code or error correction code (e.g. CRC or parity)—these additional bits can also be used by the receiver in order to correct transmission errors using mathematical methods. Furthermore, this makes it more difficult for a potential (unauthorized) eavesdropper to identify the actually transmitted data (useful data). The bandwidth is obtained from the modulation type and the symbol rate. The quantity of supplementary bits required for setting the desired bandwidth is obtained from the modulation alphabet and the clock rate of the modulator.        
The supplementary data (supplementary bits) can be positioned in the overall data stream by means of one of the following alternatives:                inserting the supplementary data after the useful data—in this case, however, the useful data can be extracted relatively easily by unauthorized third parties;        alternately stringing together useful data and supplementary data, i.e. n bits of useful data, z bits of supplementary data, n bits of useful data, z bits of supplementary data, etc., the parameters n and z being prescribed by the controller and being able to change dynamically during the transmission in a further embodiment of the invention;        inserting the supplementary data in accordance with the two alternatives above plus additional interleaving, i.e. pseudo randomly permuting, the overall data in order to hamper or prevent extraction of the useful data by unauthorized third parties.        
In a further form of the invention, the bandwidth can also be varied by virtue of the data being transmitted either in a relatively short or a relatively long transmission period for a hop, with a relatively short hop transmission period producing a relatively high bandwidth and a relatively long hop transmission period producing a relatively low bandwidth. This requires no supplementary data to be inserted into the signal. However, a correlation between bandwidth and transmission period must be accepted.
3. Varying the transmission period of a hop, wherein in one specific embodiment the pause period between the individual hops can additionally be varied as a further parameter. In this case, supplementary data can also be inserted for the useful data, with the methods cited above being able to be used, in principle.
The changes to said signal parameters are advantageously made during the switching times of the frequency switching and therefore apply for the entire duration of a hop.
The signal parameters can be changed for each new hop, but this is not necessary. By way of example, provision may also be made for a change to be made only for each second or third hop. Clearly, any other sequences of hops at which changes are meant to be made are also possible in this case. Pseudo random change patterns are likewise advantageous in this context in order to hamper classification.
As a particular advantage, the following changes to signal properties are combined with one another:
a) changing the modulation type and changing the bandwidth.
b) changing the modulation type and changing the transmission period of a frequency hop—in addition to changing the transmission period, it is also possible for the pause period between two directly successive frequency hops to be combined with changing the modulation type.
c) changing the bandwidth and changing the transmission period of a frequency hop—in addition to changing the transmission period, it is also possible for the pause period between two directly successive frequency hops to be combined with changing the bandwidth.
The switching between the individual modulation types and the bandwidths and the adjustment of the transmission period of a hop are effected using a pseudo random pattern that can be generated using a key generator, for example (for example using a linear feedback shift register). This pattern needs to be the same for the transmitter and the receiver and is advantageously chosen such that correlations between modulation type, bandwidth, transmission period of the hop and frequency range are minimized.
Without limiting the general nature, the method according to the invention can be used not only for transmitting voice but also for general data transmission (any bit streams).
The invention has the following advantages:                Avoidance of de-hopping and of consequently possible decoding of the signal using the parameters signal bandwidth, power density distribution or modulation type. As a result, potential de-hopping now has only the direction (and possibly also the signal strength) available as a single selection criterion. However, as soon as the frequency hopping range of the transmitter is larger than the scanning bandwidth of a direction finder, then although the direction of the individual transmission packets could be determined they would at best be perceived as various bursts from a particular direction.        Modulation type recognition units require a certain time in order to robustly recognize a modulation type. Although technological progress means that this period of time is becoming increasingly shorter, a rapid change—as made possible with the method according to the invention—makes recognition impossible while the (arbitrarily adjustable) transmission period of the frequency hop is shorter than the time required for recognition.        The high dimensionality of the adjustable parameters hampers the entry of such a radio in an emitter database or makes it impossible. This in turn hampers or makes impossible automatic identification of the radio.        It is not possible to make out explicitly how many radios/transmissions there are.        